1. Field of the Invention
The invention relates to a floating lowering and lifting device comprising a floating structure and a lifting unit lowerable from the floating structure towards the sea bed, the lifting unit having a chamber and a releasable coupling member for releasably attaching to a load, the chamber being connected via a lifting cable to a take up device on the floating structure for lengthening and shortening the lifting cable, the lifting unit comprises a weight balancing member attached with an upper end to the lifting cable and with a lower end to the chamber.
2. Description of Related Art
It is known to lower large weight loads (templates for example) onto the seabed with cables from a floating barge. A problem with prior art systems which use a tensioned connection between the weight and the floating vessel, like a cable, to take the weight, is that due to the movements of the floating vessel snap tensions will be introduced in the cable.
As very long cables and very large weights are used, these snap tensions can break the cable (this problem is solved by the construction according to U.S. Pat. No. 5,190,107, a heave compensating support system for positioning a sub sea work package). In very deep waters and with very large weights, the diameter and the weight of the cables are becoming to big to handle: for example the weight of a 6 inch cable of 1000 m is about 100 tons and the diameter of the cable will be to big to handle. It is possible to use devices to lower packages onto the seabed with the help of pressurized closed buoyancy cans. The cans must be so constructed to withstand the water pressure at seabed level; every 10 m water depth will add 1 bar. Such a system is shown in the above U.S. Pat. No. 5,190,107. The buoyancy can of U.S. Pat. No. 5,190,107 comprises a heave compensating system formed by a chain part at the end of the lifting cable, the lower end of the chain part attaching to the bottom of the buoyancy can. The natural frequency of the vessel at the sea surface is thereby decoupled from the motions of the buoyancy can. By varying the distribution of the length of the chain that depends in a loop from either the lifting cable or the buoyancy can, the trimming of the buoyancy is adjusted and the speed of raising and lowering can be varied. It was found that the construction wherein the catenary chain is situated on one side of the buoyancy can results in difficulties when maneuvering the buoyancy can from the surface vessel in a lateral direction (parallel to the sea bed). Furthermore, upon closer approach of the buoyancy can to structures on the sea bed, the pending chain can be in the way and may collide with the sub sea structures unless sufficient distance is maintained.
Very deep waters have relative high pressures at seabed level. This, combined with the relatively large weight to be transported makes the use of closed buoyant cans or modules very expensive due to the size of such a buoyancy module and the construction needed to avoid collapsing of the buoyancy module.
It is therefore an object of the present invention to provide a lowering and lifting device for lifting our lowering relatively heavy weights in deep water.
It is a further object of the present invention to provide a lowering and lifting device which can be raised and lowered in a controlled manner using a simple and reliable control system.
It is a further object of the present invention to provide a lowering a lifting device which can be accurately maneuvered above the sea surface, in particular in a lateral direction.
It is again an object of the present invention to provide a lowering and lifting device which can be brought into close proximity to the sea bed or to a structure on the sea bed, without interference of the weight balancing member.
It is another object of the present invention to provide a lowering and lifting device which can maintain an accurately defined volume of air in its interior.